In some processes used in the semiconductor industry, it is desirable to heat a substrate rapidly to reduce the portion of the thermal budget used by a process. Typically, rapid thermal processing systems utilize a high intensity light source to rapidly heat a substrate that is held within a processing chamber, sometimes under vacuum conditions. The light source, which may consist of an array of high intensity lamps, is located inside the chamber or outside of the chamber and adjacent to a transparent window through which the light passes into the chamber. Inside of the chamber the substrate is supported with very little physical contact (usually around the edge) so the substrate temperature can respond quickly to the incoming light. The front of the wafer is exposed and receives light from the high intensity lamps. The lamps are essentially black-body radiators and are heated as quickly as possible (typically 300 to 500 ms) to operating temperature. For many substrates, like silicon substrates as commonly used in the manufacture of integrated circuits, optical absorption is higher for shorter wavelengths especially at the beginning of a heating cycle when the substrate is closer to room temperature. Rapid silicon substrate heating begins after the lamps reach high temperatures (about 3000° C.) at which time the lamps begin emitting a significant portion of short wavelength light.
FIG. 1 shows a schematic cross-sectional view of a flood type rapid thermal heating apparatus in which a wafer 100 disposed in chamber 105 is heated by radiation from lamps 125 mounted on a chamber lid 120. The lamps 125 are typically tungsten-halogen lamps and may be brought to different temperatures to evenly heat the substrate. Pyrometry measurements may be made by monitoring light through windows 135 in the chamber 105. The rate with which the lamps 125 can be turned on and off is limited with typical heat lamps and results in limitations on how fast a substrate can be heated.
Alternative light sources have been used to overcome some of these limitations and to provide shorter pulse durations in order to stay within processing time targets. Annealing substrates, or their near-surface regions, with a flash lamp provides optical pulse durations from about 100 microseconds to 1 milliseconds and pulsed laser processing has been used to provide optical pulses between about 1 nanosecond about 100 nanoseconds. For pulses of short duration, spatially inhomogeneous pulses result in poor process uniformity across an illuminated portion of a substrate.
Since illumination may modify the substrate by thermal means, inhomogeneous pulses that cause thermal non-uniformities can significantly impact wafer yields. Some examples of how substrates are modified by thermal means include diffusion of dopants, exposure of photoresist, and chemical alterations or reactions that form a film on the substrate.
Therefore, a system and method for reliably and rapidly thermally processing wafers with improved uniformity is needed.